1. Field of the Invention
The present invention relates to a circuit pattern exposure method for forming circuit patterns on a semiconductor substrate and to a mask that is used in this method.
2. Description of the Related Art
With technological advances in the field of photolithography, micro circuit patterns can now be formed having a pitch that is less than one-half the optical wavelength of the exposure light. In particular, in the formation of a dense pattern such as a line/space pattern (hereinbelow abbreviated as “L/S pattern”) in which lines and spaces are repeated at a fixed pitch, sufficient depth of focus is obtained by the application of an oblique-incidence illumination method. The oblique-incidence illumination method is a method in which the vertical-incidence component of the illumination light that is irradiated onto a mask is cut such that the mask pattern is illuminated by the oblique-incidence component. In normal image formation, three light beams from the mask pattern, i.e., the 0-order diffracted light and the “+” and “−” first-order diffracted light, are condensed by a projection lens (image formation by three-beam interference). In contrast, in oblique-incidence illumination, one of the ± first-order diffracted lights is discarded, and two light beams, the 0-order diffracted light and one of the ± first-order diffracted lights, are condensed by a projection lens (image formation by two-beam interference).
When comparing the best-focused states of image formation realized by three-beam interference and image formation realized by two-beam interference, the image contrast in image formation by two-beam interference is reduced, because the extent of the “+” or “−” first-order diffracted light has been discarded. However, in two-beam interference, the angle of incidence on the image-formation plane (the semiconductor substrate) is one-half that for three-beam interference. As a result, the degree of blurring when out of focus is less in two-beam interference imagery than in three-beam interference imagery. Sufficient light intensity distribution can therefore be obtained in circuit pattern formation over a broader range of focus. In addition, it is known that the use of a half-tone phase-shift mask can extend the depth of focus (the range of focus over which circuit patterns can be formed). Here, a half-tone phase-shift mask is a mask in which a shield region is formed on a mask to semi-transparency (transmittance of 2-20%), whereby the phase of light that passes through the shield region is rotated 180° with respect to the phase of light that passes through the non-shielded region around the periphery of the shield region.
The use of a half-tone phase-shift mask and an oblique-incidence illumination method in the formation of an S/L pattern that produces diffracted light can improve the balance between the 0-order diffracted light and the “+” first-order diffracted light (or the “−” first-order diffracted light) and can further improve contrast.
On the other hand, a modified illumination method such as oblique-incidence illumination has little effect on an isolated pattern in which diffracted light does not occur, and the depth of focus is not greatly extended. Illumination optics having lower NA or lower coherence are more effective for extending the depth of focus of an isolated pattern. Here, illumination optics having lower NA means that the mask pattern is only illuminated by light that is close to the vertical component. Even when using a half-tone phase-shift mask, low-coherence illumination extends the depth of focus. Essentially, it has been difficult to simultaneously improve the exposure characteristics of both an isolated pattern and a highly concentrated pattern.
However, a method has been investigated in which a mask is used that is provided with mask patterns referred to as “auxiliary patterns” which do not themselves directly contribute to the formation of circuit patterns. Auxiliary patterns are disclosed in JP-A-H04-268714 (third page, FIG. 4(a)(b)). According to this publication, when using an oblique-incidence illumination method to form micro hole patterns and slit patterns, the auxiliary patterns are arranged on the mask to extend the depth of focus. The document further discloses that a similar effect can be obtained when forming line patterns. Essentially, the publication shows that when a mask, in which main patterns corresponding to the intended circuit patterns and the above-described auxiliary patterns are arranged, is used under modified illumination conditions, the image formation state approaches the state of image formation in two-beam interference and the depth of focus is extended. In this case, the position and dimensions of the auxiliary patterns greatly influence the depth of focus.
The optimum value of the spacing between the auxiliary patterns and the main patterns varies according to the dimensions of these patterns and optical conditions. Typically, the optimum value is approximately 1.5 times the limiting resolution of the optical conditions. JP-A-H05-2261 (third page, FIG. 1) discloses that the combined use of the oblique-incidence illumination method and a half-tone phase-shift mask can improve the decrease in contrast in two-beam interference image formation. In two-beam interference image formation, the 0-order diffracted light having information relating to the average brightness is excessively stronger than the “+” first-order diffracted light (or the “−” first-order diffracted light) having information relating to pitch. As a result, the amplitude of the distribution of the intensity of exposure light becomes smaller than the average value, and the contrast drops. Thus, using a half-tone phase-shift mask to reduce 0-order diffracted light improves the balance of light intensity and suppresses a decrease in contrast.
As described in the foregoing explanation, when the oblique-incidence illumination method was used, a mask was used in which auxiliary patterns were arranged at the periphery of concentrated patterns in which the effect of extending the depth of focus was difficult to obtain. More accurately, exposure was carried out using a mask in which auxiliary patterns were arranged at the periphery of the main patterns that corresponded to circuit patterns that were concentrated. As a result, the outermost patterns of concentrated circuit patterns could be realized in the desired dimensions. Alternatively, the desired depth of focus was obtained.
However, in the prior art, no consideration was given to the dimensional accuracy of the patterns that were inward from the outermost portion of the patterns when forming concentrated circuit patterns. The inventors of the present invention have discovered that even when the desired dimensions are realized for the outermost portion of concentrated circuit patterns, the dimensions of patterns within the outermost portion tend to vary.